1. Field of the Invention
This invention relates to an image display apparatus, provided with an envelope having two substrates opposed to each other and a plurality of image display elements arranged inside the envelope, and a method of manufacturing the same.
2. Description of the Related Art
Recently, various flat display apparatuses have been developed as a next generation of lightweight, thin image display apparatuses to replace cathode-ray tubes (hereinafter referred to as CRT). These flat display apparatuses include a liquid crystal display (hereinafter referred to as LCD), plasma display panel (hereinafter referred to as PDP), field emission display (hereinafter referred to as FED), surface-conduction electron emitter display (hereinafter referred to as SED), etc. In the LCD, the intensity of light is controlled by utilizing the orientation of liquid crystals. In the PDP, phosphors are caused to glow by means of ultraviolet rays that are produced by plasma discharge. In the FED, phosphors are caused to glow by means of electron beams that are emitted from field-emission electron emitting elements. In the SED, phosphors are caused to glow by means of electron beams that are emitted from surface-conduction electron emitting elements.
In general, the FED or SED, for example, has a front substrate and a rear substrate that are opposed to each other with a given gap between them. These substrates have their respective peripheral portions joined together by means of a sidewall in the form of a rectangular frame, thereby constituting a vacuum envelope. A phosphor screen is formed on the inner surface of the front substrate. A large number of electron emitting elements for use as sources of electron emission that excite the phosphors to luminescence are provided on the inner surface of the rear substrate.
In order to support an atmospheric load that acts on the front substrate and the rear substrate, a plurality of support members are arranged between the substrates. The potential on the rear substrate side is substantially equal to the earth potential, and an anode voltage is applied to the phosphor surface. Electron beams that are emitted from the electron emitting elements are applied to red, green, and blue phosphors that constitute the phosphor screen, whereupon the phosphors are caused to glow, thereby displaying an image.
According to the FED or SED constructed in this manner, the thickness of the display apparatus can be reduced to several millimeters. Therefore, it can be made lighter in weight and thinner than a CRT that is used as a display of an existing TV set or computer.
In the FED or SED described above, a high vacuum must be formed in the envelope. Also in the PDP, the envelope must be evacuated once before it is filled with discharge gas.
As means for evacuating the envelope, there is a method in which the front substrate, rear substrate, and sidewall that constitute the envelope are heated and joined together by means of a suitable sealing material in the atmosphere. After the envelope is then exhausted through an exhaust pipe that is attached to the front substrate or the rear substrate, in this method, the exhaust pipe is vacuum-sealed. In evacuating the flat envelope through the exhaust pipe, however, the exhaust speed is very low, and the attainable degree of vacuum is low. Thus, the mass-productivity and properties are not reliable.
A method to solve this problem is described in Jpn. Pat. Appln. KOKAI Publication No. 2000-229825, for example. According to this method, the front substrate and the rear substrate that constitute the envelope are finally assembled in a vacuum chamber.
In this method, the front substrate and the rear substrate that are brought into the vacuum chamber are first fully heated in advance. This is done in order to reduce the gas discharge through the inner wall of the envelope that constitutes the principal cause of lowering of the degree of vacuum of the envelope. When the front substrate and the rear substrate are then cooled so that the degree of vacuum in the vacuum is fully enhanced, a getter film for improving and maintaining the degree of vacuum is formed on the phosphor screen. Thereafter, the front substrate and the rear substrate are heated again to a temperature high enough to melt the sealing material. The front substrate and the rear substrate are combined together in a predetermined position as they are cooled so that the sealing material is solidified.
For the vacuum envelope constructed by this method, a sealing process doubles as a vacuum encapsulation process. Besides, a lot of time that is required for exhausting can be obviated, and a very satisfactory degree of vacuum can be obtained. Preferably, in this method, moreover, a low-melting-point metallic material that is suited for batch sealing and encapsulation should be used as the sealing material. Since the low-melting-point metallic material has low viscosity when it is melted, however, it may possibly flow out of a desired sealing region during the sealing operation.
Flat display apparatuses such as the SED, in particular, require a high degree of vacuum, and inevitably become defective if the sealing layer allows a single leakage. In order to improve the yield of manufacture or mass production of large-sized image display apparatuses, therefore, the airtightness of the sealing portion must be enhanced to ensure higher reliability.